Basement Ice, Ward Hunt Ice Shelf , Ellesmere Island

Journal o/Glaciologv, Vol. 10, No. 58,1971

BASEMENT ICE, WARD HUNT ICE SHELF, ELLESMERE ISLAND, CANADA

By J. B. LYONS (Department of Earth Sciences, Dartmouth College, Hanover, ew Hampshire 03755, U .S.A.)

S . M . SAVIN (Case W estern Reserve University, Cleveland, Ohio, U.S.A.)

and A. J. TAMBURI (Colorado State University, Fort Collins, Colorado 80521, U.S.A.)

ABSTRACT. Oxygen-isotope a nd chlorinity d eterminations, as well as petrographic observations, indicate that the basement ice of the Ward Hunt Ice Shelf is largely composed of a unique brackish ice, w hich interdigitates with sea ice. Some iced firn occurs near the top of the basement ice, below an unconformity. Stratification in brackish a nd sea ice represents annual increments to the bottom of the ice shelf. The c-axis vertical ori enta tion and small-angle grain-boundary relations in brackish ice a re explained by nucleation and fl oating of ice d endrites from the undercooled brackish water zone to the bottom of the ice shelf, where they attach themselves sub-para ll el to the plane of the undersurface. Ice isla nd T-3 did not come from a break-up of the main part of the "~Yard Hunt Ice Shelf but probably origina ted in a nearby a rea to the west. RESUME. Glace defond dallS la Ward H unt Ice Shelf, Ellesmere IslalId, Canada. Des determinations d e teneur en isotopes d e l'oxygene et d e chlorinite, a insi que des observations petrographiques montrent que la glace de la base d e la W a rd Hunt Ice Shelf est en majorite composee d 'un type unique de glace sauma tre avec intercala tions de doigts d e glace d e mer. II a rrive que du neve passant a la glace se trouve pres du sommet de la glace de base, a u pied d 'une irregularite. La stra tification dans la glace saumatre et dans la glace de mer represente I'accroissement a nnuel par le fond d e la banquise. L'orienta tion verticale d es axes c et les lia isons a angle fa ible a ux limites d e grains d a ns la glace sauma tre sont expliques par la nucleation et le flottage de dendrites de glace a partir de la zone d 'eau saumatre en surfusion en dessous de la ba nquise, a laquell e eUes se fixent a peu pres pa ra Jl element all plan d e I'interface glace-eau. L'ile de glace T-3 ne vient pas d 'une rupture de la partie principale d e la Ward Hunt Ice Shelf mais a probablement pris naissance dans une region voisine a l'Ouest. ZUSAMMENFASSUNG. Eis an der U'llerseile des Ward H unt Ice Shelf, Ellesmere Island, Kanada. Sauerstoffisotopen und Chlorgeha ltsbes timmungen wie auch petrographische Beobachtungen weisen d a ra uf hin, dass das Eis an der U nterseite des Ward Hunt Ice Shelf sich weitgehend aus einem eigenartigen brackigen Eis zusammensetzt. das mit Meereis verzahnt ist. Nahe seiner oberen Grenze tritt unterha lb ein er Unstetigkeit etwas vereister Firn auf. Die Stratigraphie des Brackeises und des M eereises spiegelt den jahrlichen Zuwachs an d er Unterseite des Schelfeises wied er. Die vertikale Orientierung der c-Achsen und die kleinen Winkel zwischen den K orn grenzen im Brackeis werden durch K ernbildung und Aufschwimmen von Eisdentriten aus der unterkuhlten Brackwasserzone an die Schelfeisunterseite erklart, wo sie sich parallel zur Unterseite anheften. Die Eisinsel T-3 ist nicht durch Abbrechen vom H auptteil des Ward Hunt Ice Shelf entstanden, sondern stammt vermutlich aus einem westlich nahe benachbarten Gebiet.

INTRODUCTION The ice islands of the Arctic O cean on gmate from the break-up of ice shelves, such as the Ward Hunt Ice Shelf, along the northern coast of Ellesmere Isla nd. As calculated from mean freeboard and ice-density relations, the Ward Hunt Ice Shelf has an average thickness of 44 m. It is composed of two major stratigraphic units: (I) a lower "basement" ice thought by Marshall (1960, p. 47 ) to be largely of glacial origin and brine-soaked, but by Nakaya and others (1962, p. 29 ) to be of a quite uncertain origin, and (2) an upper unit of inter stratified iced firn and la ke ice lying with angular unconformity (Marshal! , 1960; Lyons and Leavitt, 1961 ) upon the basement ice. The unconformity is easily mapped at the surface or is recognized in drill COI"es because it is marked by a heavy concentration of aeolian dust developed during an ablation cycle which separates the times of formation of the lower and upper shelf stratigraphic uni ts (Fig. I).

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Downloaded from https://www.cambridge.org/core. 29 Sep 2021 at 10:58:17, subject to the Cambridge Core terms of use. WARD H U NT I C E S HELF , ELLESMERE I S LAND 95 This picture of the stratigraphy of the ' I\fard Hunt Ice Shelf is, to som e extent, an over simplification. Radio echo-sounding (Evans and others, 1969; Hattersley-Smith and others, 1969) suggests that the ice shelf may have a total range in thickne s of from 25 to 80 m . Cores from the northern edge of the Ward Hunt ice rise and from the shelf north-east of the ice rise (Ragle and others, 1964) show no heavy dust layer nor any basement ice; instead. the lower 20- 25 m consist of sea ice. Thus the outer part of the ice shelf, a large part of which calved off in 1961 - 62 (Hattersley-Smith, 1963; NUlt, 1966), has a sea-ice basement which differs significan tly from the inner basem en t ice de cri bed originally by Mar hall (1960) for the Ward Hunt I ce Shelf, and by Nakaya a nd others (1962) on ice island T -;l . The petro graphic recognition of a difference in the nature of the ice of the lower part of the W ard Hunt I ce Shelf in its inner and outer parts is a lso consistent with Crowley's ( 1961) seismic studies of the ice shelfwhich showed acoustic differences in the basement ice in these two place. . It is the inner' basement ice which is the chief su bj ect of the present communication.

PETROGRAPllY OF I3ASEMENT ICE Most of the inner basement ice, where exposed, shows a remarkably well -developed stratification (Fig. 2) reAecting cyclic variations in the abundance of entrapped gases and average grain-size of the ice. On both T -3, where it consti tutes a major part of the ice island. and on the Ward Hunt I ce Shelf, the strata a re 20-25 cm in average thickness, and are locally warped into a series of folds with wavelengths of the order of 100 m , and dips on the limbs of as high as 90° (Lyons and Leavitt, 196 1, p. 14- 19; Nakaya and others, 1962, p. 12; . T exturally, much of the stratified ice is characteri zed by what appear to he very large columnar crystals with indistinct grain diameters of up to 120 cm. The large crys tals, however, consisl of mosaics of smaller sub-columnar grains showing a sub-parallel orientation of theil' c -ax e~ normal to the stratification planes, and small-angle grain-boundary relations to one another (Nakaya and others, 1962, p. (3). Salinity in stratified basement ice is extremely low (Table I ; Nakaya and others 1962, fig. 28). This fact, as well as the optic ori entation (c-axis vertical

Fig. 2. Stratified basement ice . W ard HI/Ilt Ice Shelf.

Downloaded from https://www.cambridge.org/core. 29 Sep 2021 at 10:58:17, subject to the Cambridge Core terms of use. 96 JOURNAL OF GLACI0LOGY rather than horizontal) and the lack of a characteristic platelet substructure, effectively rule out the possibility that the stratified ice could be sea ice recrystallized in situ. Similar petro graphic arguments may be marshalled against iced firn, glacial ice or lake ice as possible prototypes of the stratified basement ice. Not all of the basement ice is of the stratified type. Iced firn has been identified in the upper basement ice of both the Ward Hunt Ice Shelf and T-3 by Lyons and Leavitt (1961 , p. 8) and by Nakaya and others (1962, p . 20), respectively. The latter investigators also identified 2 m of sea ice at the bottom ofT-3. In holes drilled into Ward Hunt basement ice during the 1969 field season (cr. Fig. I) at locations 1.6 and 2.6 km south-west of the south western tip of the island, stratified basement ice and sea ice alternated in the core samples in the manner shown in Figure 3. Identification of ice types was based upon petrographic criteria in the field and was confirmed in the laboratory by chemical tests (Table I ).

TABLE I. (jISO AND CHLORI NITY DETERMINATION ON ICE AND WATER SAM PLES, NORTHERN ELLESMERE ISLAND

OISO Sample D epth (relative to Number of Average ChloriTlil)' number m Petrographic description SMO W standard) analyses deviation 0 / 00 2 6 Stratified basement ice - 2 1.56 3 0.07 0. 16 3 15 Stratified basem ent ice - 13.22 3 0.23 0.19 4 18 Sea ice - 0.63 2 0.11 1. 25 5 2 1 Basement ice, transition type - 17·47 2 0.04 0.13 6 5 Fresh water, Disraeli Fiord - 27. 15 3 0.15 < 0.01 7 43 Brackish water, Disraeli Fiord - 4.66 2 0.02 16.25 8 300 Salt water, Disraeli Fiord + 0-4 1 2 0.03 17.65 9 2 Fresh water, Ward Hunt Lake - 24.06 3 0. 15 < 0.01 12 14 Iced firn, Camp Creek ice rise - 30.60 3 0.06 < 0.01 13 Teed firn, Ward Hunt ice rise - 27.29 3 0.08 < 0.01 ISO analyses by S. M. Savin. Cl analyses by L. M. Banos.

OCEANOGRAPHIC OBSERVATIONS Along the north, east and south sides of Ward Hunt Island, and along the north shore of Ellesmere Island, fresh-water moats open up during the summer melt season. In the moat between ElIesmere Island and the Ward Hunt Ice Shelf (Lyons and Leavitt, 1961, fig. 4), fi'esh water extends to a depth of 48 ± 3 m , which is a few meters below the bottom of the ice shelf. In the adjoining Disraeli fiord , K eys and others ( 1969) have shown that the upper part of the fiord is filled with fr esh water to a depth of 44 m. Between 44 and 45 m is a brackish water zone, below which is Arctic O cean sea-water with a salinity of 32%0' The ice shelf apparently acts as a dam for the surface m elt waters draining from northern Ellesmere Isla nd, but interchange with Arctic O cean water can occur at greater depths (K eys and others, 1969, p. 7). Today, as in the past, ice accreting below the Ward Hunt Ice Shelf could conceivably be fresh, brackish or saline ice, depending upon geographic location and climatologic factors (i. e. the depths of the fres h, brackish and oceanic zones relative to the ice-shelf bottom). Thinning of the ice shelf during an ablation cycle should increase the likelihood of accreting fresh water or brackish ice on its underside; thickening of the ice shelf during an accretionary cycle would enhance the probability for sea-ice accretion. In any case, complex inter-tonguing relations of ice types are to be expected. Schwarzacher (1959) has shown that accretion of fresh ice beneath thin sea ice is a common consequence of run-off of melt waters from the pack ice of the Arctic O cean, and Cow and others (1965) have described a similar situation in a part of the McMurdo Ice Shelf of Antarctica.

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< 0 :u'" -4 (') ~ r (I) (') ~ r

'" 01 ~ 0 Approximate Shelf Base Interface o. 0.5. HORIZONTAL SCALE (km,)

Fig. 3. Interpretation of stratigraphy of part of Ward Hunt Ice Shelf, based on drill-core and laboratory studies. See Figure I f or location of holes A and B. Ward Hunt ice rise to wards right side of section ; trough of syncline between Ward Hunt and Ellesmere Islands to left of section .

OXYGEN-ISOTO P E A ND CHLORINIT Y D ATA T able I presents oxygen-iso tope and chlorinity data of ten samples collected during the 1969 fi eld season. Three of them (Nos. 6, 7 and 8) were taken by J. K eys a nd H. Serson from an oceanographic station in Disraeli Fiord, and the remainder were collected by the writers. Determinations of Sl 80 were made using techniques described by Epstein and M ayeda (1953). Chlorine was determined using ion-sensitive electrodes. The basement-ice samples come from the holes drilled 1.6 and 2.6 km south-west of the south-wes t edge of Ward Hunt Island (Fig. I). The extremely low SI80 values of the ice from the Camp Creek and Ward Hunt ice rises (Nos. 12 and 13) are consistent with the petrographic a nd stratigraphic identification of iced firn. Because of the number of samples (two), we can draw no conclusions concerning the significance of the differences in SI80 values with respect to temperature of accumulation, or other factors. For the fr esh-water samples ( os. 6 and 9) the lower 8180 value from Disraeli Fiord is consistent with the fact that it receives melt wa ters from elevations of up to 1 980 m , and its average SI80 value should therefore be more nega tive than tha t from Ward Hunt Lake, which receives its drainage from elevations lower than 415 m . Lake ice forming from the melt wa ter in Ward Hunt Lake should have SI80 values of about - 24 or more positive; that forming on the Ward Hunt Ice Shelf pres umably would have values in the range of - 27 to - 30. Because the ice petrographically identified as sea ice (No. 4) has a SI8 0 value very close to 0.00, as does the Arctic O cean water (No. 8), there seems to be li ttle doubt that this ice type is identifiable both chemically and petrographicall y with considera ble certainty. The three basement ice samples (Nos. 2, 3 a nd 5) show SI80 values more negative than the brackish wa ter in Disraeli Fiord (No. 7) but not as negative as fresh melt water in the same fi ord. The SI8 0 and chlorinity of samples 3 and 5 imply crystallization of ice from brackish water. Sample 2 seems to have formed from water which was also brackish but its SI80 value is anomalously low. This sample was taken quite close to the surface in an area from which iced firn has been ablated only recently. I t is possible, though not indicated by any other criteria, that a decrease in the SI80 value has occurred because of downward percolation by firn melt water. The alternative possibility is tha t the ice crystallized from water somewhat fresher than that represented by samples 3 and 5. 4

Downloaded from https://www.cambridge.org/core. 29 Sep 2021 at 10:58:17, subject to the Cambridge Core terms of use. 98 JOURNAL OF GLACI0LOGY An obvious conclusion concerning samples 3 and 5, and probably sample 2 as well, is that they represent a newly recognized type of ice, best termed brackish ice, which is a unique but volumetrically significant constituent of Arctic ice shelves and ice islands. Crary (1960, p. 33) had suggested that the freezing offresh or brackish waters on the underside ofT-3 and the Ward Hunt Ice Shelf might have been important in their development. What seems surprising, as gauged by Figure 3, is that approximately half of the basement ice is of brackish origin.

DISCUSSIOK Tritium measurements in Disraeli Fiord (Keys and others, 1969, p. 4) suggest that each season's melt water sinks to the interface between the salt and fresh water where it may flow directly out of the fiord, under the ice shelf. However, because the ice shelf thins by ablation, particularly near the edges, there is the possibility that some of the melt water, brackish water, or sea-water may accrete as ice on the underside of the ice shelf. The brackish water is of interest in this regard, because its salinity lies in the range where theory predicts (Weeks and Lofgren, 1967) that ice growing in the sub-surface should have the c-axis horizontal orientation and platelet substructure characteristic of sea ice. Supercooling and ice-dendrite formation, as observed in the Disraeli Fiord brackish water zone (Keys and others, 1969, p. 3), provide a mechanism for explaining the c-axis vertical and small-angle grain-boundary texture so common in the brackish ice. Nucleation followed by floating of dendrites to the underside of the ice shelf, with consequent c-axis vertical preferred orientation, would seem to be an inevitable consequence of undercooling in the brackish water zone. On the other hand, there is no evidence for supercooling of the Arctic Ocean sea-water, and shelf sea ice does, predictably, have its c-axis horizontal. The concentration of bubbles marking the stratification planes in the basement ice can be understood in terms of the inverse relation between air solubility and salinity in water (Dorsey, 1940, p. 534- 49). If brackish ice accretes below the ice shelf, it will expel downward water more saline than the relatively pure ice which is crystallizing, and a condition will eventually be reached where exsolution of gas becomes inevitable. This gas concentrates beneath the year's annual increment of brackish ice and is partly trapped in the downward encroaching ice front. Flooding of the next summer's fresh or brackish water under the ice shelf partly dissolves the preceding winter's gases and allows the cycle to repeat itself. If the average stratum of basement ice is 20 cm thick, and if this represents an annual increment to the basal edge of the ice shelf because of ablation at the surface, only 220 years as a minimum might be required to bring stratified ice from the bottom of the 44 m thick ice shelf to the surface. This is a surprisingly short time but not inconsistent with a radio carbon age of 400± 150 years which Crary (1960, p. 43) reported for a siliceous sponge which had passed through the basement ice south of Ward Hunt Island. An extension of the model proposed here for the stratified basement ice is that the indi vidual strata may, on the average, thin or wedge out completely toward the center of the ice shdf. The thickness of the average layer and its nature (brackish ice or sea ice) will depend on hydrologic factors which are, in turn, related to climatology. If the ice shelf maintains a nearly constant thickness, curling up at its edges and, over the past several hundred years, accumulating firn and lake ice in the area between Ward Hunt and ElIesmere Islands, bottom melting is also a necessity under this median part of the ice shelf (Lyons and Ragle, 1962, p. 93- 94). Similarly, accretion of ice under some parts of the McMurdo and Ross Ice Shelves of the Antarctic, and ablation under others, has recently been demonstrated by Gow and others (1965) and by Swithinbank (1970). An interesting feature of the basement ice is its age relative to the upper iced firn and lake ice of the ice shelf. Some of it, immediately below the dust-marked unconformity, must

Downloaded from https://www.cambridge.org/core. 29 Sep 2021 at 10:58:17, subject to the Cambridge Core terms of use. WARD H U NT ICE SHELF , ELLESMERE ISLAND 99 be older than the superjacent iced firn and lake ice, but the lowermost basement ice must be younger than the unconformity. Hattersley-Smith and Serson (J 970) have shown that the mass balance of the Wa rd Hunt Ice Shelf and ice rise has been negative over the past decade, so the present surface is working its way downward into older iced firn and lake ice. In the ice shelf as a whole, therefore, the age increases both from the top downward, and from the bottom upward. Our interpretation of the stratigraphic-structural relations of the basement ice of the Ward Hunt Ice Shelf is shown in Figure 3. Beneath the heavy-dust ablation unconformity, the basement is gently wa rped into a broad syncline between Ward Hunt and Ellesmere Islands (cr. Fig. I ). Where the basement ice re-appears at the surface near Ellesmere Island, it is deformed into anticlines and synclines, probably because of stresses exer ted on this edge of the ice shelf by glaciers advancing from Ellesmere Island a t a time prior to the major ablation cycle during which the heavy-dust layer was concentrated . Crary's (1960, p. 44- 46) estimate of the age of the ablation cycle, ba ed upon somewhat conflicting radiocarbon dates is I 600 years. H owever, the oxygen-isotope data of Da nsgaard a nd others (J 969) from Camp Century, Greenland, suggest that a major a bla tion cycle in northern latitudes termina ted as recently as 850 years ago. Finally, the distribution and structure of the ice types comprising the Ward Hunt I ce Shelf (Fig. I ) make it clear that T-3 could not have origina ted from the area between M cClintock Fiord and Markham Bay. M cClintock Fiord itself, however, or one of the bays to the west such as Yelverton Bay remain likely sites for its point of origin (cr. Stoiber and others, 1960).

A CKNOW LEDGEMENTS W e a re indebted to V .S. Army C.R .R .E.L. a nd the Canadian Defence R esearch Board for logistic support. Dr G. H attersley-Smith, J. K eys and H. Serson of the Defence R esearch Board, and M r Bruce Petrie were most helpful in the fi eld studies . "Ve are grateful to A. J. Gow for comments and corrections to the manuscrip t. Fina ncial support for isotopic analyses was provided by N.S.F. Grant GA- I693.

M S. received 8 July 1970

REF ERENCES

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Hattersley-Smith, G., and Serson, H. 1970. Mass balance of the Ward Hunt ice rise and Ice Shelt: a 10 year record. Journal of Glaciology, Vo!. 9, No. 56, p. 247- 52. Hattersley-Smith, G., and others. 1969. G lacier depths in northern Ell esmere Island: airborne radio echo sounding in 1966, by G. Hattersley-Smith, A. F uzesy and S. Evans. Canada. Defence Research Board. Technical Note No. 69-G. Keys, ]., and others. 1969. The oceanography of Disraeli Fiord, northern Ellesmere Island, by]. Keys, O. M. ]ohannessen and A. Long. Canada. Defence Research Board. Geophysics. Hazen 34. Lyons,]. B., and Leavitt, F. G. 196 1. Structural and stratigraphic studies on the Ward Hunt Ice Shelf. Bedford, Mass., Geophysics Research Directorate, U.S. Air Force Cambridge Research Laboratories. (Final report on Contract AF 19 (604)-6188.) Lyons,]. B., and Ragle, R. H. 1962. Thermal history and growth of the Ward H unt Ice Shelf. Union Giodisique et Giophysique Internationale. Association Internationale d'Hydrologie Scientifique. Commission des Neiges et Glaces. Co[[oque d'Obergurgl, [0-9- [8- 9 [962, p. 88-97. Marshall, E. W. 1960. Structure and stratigraphy of T -3 and the Ellesmere ice shelf. (In Bushnell, V. C., ed. Scientific studies at Fletcher's ice island, T-3, 1952- [955. Vo!. 3. Bedford, Mass., Geophysics Research Directorate, U.S. Air Force Cambridge Research Center, p. 45- 57. (Geophysical Research Papers, No. 63.)) Nakaya, U ., and others. 1962. Glaciological studies on Fletcher's ice island (T-3), by U. Nakaya,]. Muguruma and K. Higuchi. Arctic Institute of North America. Research Paper o. 21. Nutt, D. C. 1966. The drift of ice island WH-5. Arctic, Vo!. 19, No. 3, p. 244-62. Ragle, R. H., and others. 1964. Ice core studies of Ward Hunt Ice Shelf, 1960, by R. H. Ragle, R. G. Blair and L. E. Persson. Journal of Glaciology, Vo!. 5, o. 37, p. 39- 59. Schwarzacher, W. 1959. Pack-ice studies in the Arctic Ocean. Journal of Geophysical Research, Vo!. 64, No. 12, p. 2357--67· Stoiber, R. E ., and others. 1960. Petrographic evidence on the source area and age of T-3, by R. E. Stoiber, J. B. Lyons, W . T . Elberty and R. H . McCrehan. (In Bushnell, V. C., ed. Scientific studies at Fletchey's ice island, T-3, 1952-[955. Vo!. 3. Bedford, Mass., Geophysics Research Directorate, U .S. Air Force Cambridge Research Center, p. 58- 72. (Geophysical Research Papers, No. 63.)) Swithinbank, C. [W. M.] 1970. Ice movement in the McMurdo Sound area of Antarctica. [Union Giodisique et Giophysique Internationale. Association Internationale d' Hydrologie Scientifique.] [International Council of Scientific Unions. Scientific Committee on Antarctic Research. International Association of Scientific Hydrology. Commission of Snow and l ee.] International Symposium 07' Antarctic Glaciological Exploration (ISAGE), Hanover, New Hampshire, U.S.A., 3- 7 September [968, p. 472-87. Weeks, W. F., and Lofgren, G. 1967. The effective solute distribution coefficient during the freezing of NaCl solutions. (In Oura, H., ed. Physics of snow and ice: international coriference on low temperature science . ... I966. ... Proceedings, Vo!. I, Pt. I. [Sapporo], Institute of Low Temperature Science, Hokkaido University, p. 579--97.)

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